Re-routing traffic in a communications network

ABSTRACT

Method of re-routing traffic in a communications network ( 1 ) in the event of a fault ( 3 ) on a path across the network, the method comprising, determining whether a first node ( 4 ), located between the fault and a network ingress node ( 6 ), is capable of switching traffic to an alternative path which avoids the fault, and if the first node is determined to be not so capable, then determining whether a second node ( 8 ), located upstream of the first node, is capable of switching traffic to an alternative path which avoids the fault.

This application is the U.S. national phase of International ApplicationNo. PCT/EP2008/058844, filed 8 Jul. 2008, which designated the U.S. andclaims priority to EP Application No. 08156047.6, filed 12 May 2008, theentire contents of each of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates generally to re-routing traffic in acommunications network.

BACKGROUND

Despite the publicity on multimedia services which require apoint-to-multipoint connectivity, like television streams multicast overInternet Protocol (IPTV), the support of E-Tree services is a recentevent in Connection Oriented-Packet Switched (CO-PS) networks, likeMulti Protocol Label Switching (MPLS), Transport MPLS (T-MPLS) orProvider Backbone Bridging Engineering (PBB-TE). The debate is stillopen on how to efficiently support E-Tree services and provide therequired network resiliency in CO-PS networks. Some doubt has beeninstilled that CO-PS networks are not really as good asConnectionless-Packet Switched (CL-PS) networks for such services.Currently, no finalized standard solution exists.

Existing connectionless options like Ethernet-based networks (e.g.Provider Bridge or Provider Backbone Bridge) still rely on very basiccontrol plane solutions for both loop avoidance and resiliency, like thefew variants of the Spanning Tree Protocol. Along with their limitedcapability to implement sophisticated traffic engineering, this is oneof the reasons why the move towards CO-PS networks is ineluctably takingplace, forcing the standard making bodies and the technical community toface the issues related to the efficient support of E-Tree services.

Ultimately, solutions which completely rely on Layer 3 of the OpenSystems Interconnection (OSI) Basic Reference Model (i.e. the IP layer)are unsuitable, cost inefficient and of inappropriate complexity,especially in relation to metropolitan networks.

Standard-making bodies are currently working on a definition of a propernetwork infrastructure to support E-Tree services with the requireddegree of efficiency. No complete interoperable solution has beenfinalized so far for point-to-multipoint infrastructures, but there havebeen several attempts to solve this problem in CO-PS networks. The mostefficient solutions make use of tree infrastructures built in the CO-PSnetwork that connect an ingress or root node (which is a node where theE-Tree service enters the network/sub-network) to the several egressnodes (which are the destinations of the E-Tree service), with the aimof optimizing the overall use of network resources. Extensive literatureis available on how to build an optimum tree for a particular networktopology and possible constraints. In addition, resiliency is a basicrequirement for this kind of service, because revenue generatingapplications, like IPTV, cannot be delivered to paying customers withpoor quality or unacceptable interruptions. This tree infrastructuretherefore needs to be protected against link or node failures, possiblyin a non-traffic consuming way, like in 1:1 or restoration schemes,where the traffic is sent onto a backup path when the primary path hasfailed.

A few known solutions address this requirement, with local repairschemes, like Fast ReRoute (FRR) in MPLS, or with global repair schemeswhose operation consist in providing a complete backup tree. Alimitation of local repair schemes is that in the case of node failurethe tree infrastructure needs to be locally modified, because some othernode in the network needs to forward the traffic in a different way tomake up for the failed node and ensure the E-Tree service trafficcontinuity. This can be difficult to implement, require a very highdegree of complexity and result in a longer recovery time. In addition,traffic duplication is possible during fault conditions. Besides theneed to actually configure a potentially very high number of alternativepaths in order to avoid issues with single points of failure, the mainlimitation of FRR lies in its necessarily local repair nature, which isnot particularly well suited for a potentially non-trivial treeinfrastructure.

In the case of global repair schemes, irrespective of where and what thefault condition is in the active tree infrastructure, all of the trafficrelated to the E-Tree service is switched to the backup tree. This canbe particularly problematic for the egress nodes, since even though theswitching time is kept to a minimum, all such nodes will see an impacton the traffic, even those which are remote from where the fault hasoccurred. To add further complexity to such a scenario, E-Tree specificcontrol plane protocols, like for instance IGMP in case of IPTV, canexperience difficulties and may need to recover updated information onthe backup tree before allowing the traffic to be forwarded normallyagain. This can lead to an even longer time for the protection scheme tofinally converge and return to normal operation.

SUMMARY

According to one aspect of the invention there is provided a method ofre-routing traffic in a communications network in the event of a faulton a path across the network. The method comprises, determining whethera first node, located between the fault and a network ingress node, iscapable of switching traffic to an alternative path which avoids thefault. If the first node is determined to be not so capable, thendetermining whether a second node, located upstream of the first node,is capable of switching traffic to an alternative path which avoids thefault.

According to another aspect of the invention there is provided acommunications network comprising a plurality of nodes which areconnected by respective links to form a path for traffic across thenetwork. The nodes comprising at least one re-routing node configured todetermine that a fault has occurred between the re-routing node and anetwork egress node, and the re-routing node configured to determine ifit is able to switch traffic onto an alternative path to avoid thefault. If it is not so able then the re-routing node configured to causea fault notification signal to be sent to an upstream node, and theupstream node configured to determine if it is able to switch traffic toan alternative path to avoid the fault.

According to a further aspect of the invention there is provided a nodefor use in a communications network. The node comprises a processorconfigured to determine that a fault in the network has occurred andconfigured to determine whether it is capable causing traffic to beswitched to an alternative path. In the event that the processordetermines that it is not so capable, the processor configured to causea fault notification signal to be transmitted for reception by a secondnode.

According to yet a further aspect of the invention there is provided amethod of configuring a communications network to provide at least onealternative path in the event that a fault occurs in a path across thenetwork. The method comprises configuring at least one re-routing nodeof the network to determine that the fault has occurred, and configuringthe at least one node to determine whether it is able to switch thetraffic to an alternative path to avoid the fault. The method alsocomprises configuring the node such that if the node determines that itis not so able then the node issues a fault notification signal toanother node.

According to another aspect of the invention there is providedmachine-readable instructions to configure a node, the instructionscomprising instructions to cause a processor of the node to determinethat a fault has occurred in a communications network in which the nodeis to be located, instructions to cause a processor to determine whetherthe node is capable of providing a switch to divert traffic onto analternative path to avoid the fault, and instructions to cause theprocessor to bring about the node issuing a fault notification signal ifthe node is determined not to be so capable

DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention will now be described, by way ofexample only, in which:

FIGS. 1 to 4 show a communications network,

FIG. 1a is a communications network node,

FIG. 5 is a flow diagram, and

FIG. 6 is a flow diagram.

DETAILED DESCRIPTION

A network 1, which is configured to implement an embodiment of thepresent invention, is shown in FIG. 1. The network 1 comprises a meshednetwork and a tree structure has been identified and configured.Broadly, the network 1 comprises a plurality of nodes, shown as circularelements, and links providing communication between the nodes, the linksshown by lines connecting the nodes. A root node, which is the noderesponsible for the distribution of an E-tree service within the network1, and may be termed a network ingress node is designated by the letter“R”. The network 1 further comprises a plurality of leaf or egressnodes, which are the nodes attached to the destinations of the E-treeservice, and are each designated by the letter “L”. Each node of thenetwork comprises a processor configured to cause received traffic to beforwarded as required. For example, a node 4 comprises a processor 80,as shown in FIG. 1 a.

The network 1 provides an active tree structure to support the E-treeservice and an associated stand-by tree structure. The stand-by treestructure is arranged to be used in what may be termed as a “fractal”manner. This means that a type of hierarchy in the active tree iscreated such that there is the possibility to identify sub-trees whichcan react autonomously to fault conditions without needing the faultcondition to be notified to the root node of the whole tree in order toswitch all the traffic related to the E-tree service onto a stand-bytree. The active tree structure and the stand-by tree structure (bothcomprising of a number of sub-trees) are identified at the time ofconfiguring the network or when the service which needs to be supportedhas to be rolled-out. Path diversity between the active and the stand-bytree structures and sub-tree structure have to be configured in order tolimit to a minimum number, or indeed eliminate, single points offailure. Two logically disjoint trees can always be identified, eachcomposed of a number of sub-trees. As will described below, in the caseof fault, the tree is “repaired” as close as possible to the faultitself, performing a protection switch operation only at the mostappropriate sub-tree level, if any, and not necessarily at thewhole-tree level (which will occur only when no sub-tree is able to dealwith the specific fault). In this way the traffic impact is confinedonly to the minimum and unavoidable number of leaf nodes, and notnecessarily to the complete tree.

FIG. 1 shows the network 1 in normal working conditions. A tree, thelinks of which are shown by emboldened arrows, provided by the network,comprises two main portions. These are a left sub-tree portion and rightsub-tree portion, as can easily be seen in FIG. 1. As will be describedbelow, some of the nodes of the tree are configured as re-routing nodesto route traffic to an alternative path to avoid a fault.

In FIG. 2 a fault condition, shown at 3, has occurred in the network 1,to a link 13 attached to a leaf node 2. This part of the tree has beenconfigured as a sub-tree, since, due to the physical topology of thenetwork 1, is capable of autonomously recovering from a range of faults,included the fault 3 shown in FIG. 2. A node 4 serves as a re-routingnode, which is the node responsible for switching the traffic from anactive sub-tree (i.e. the one used in normal operating conditions) to astand-by sub-tree, and can be termed the root of that particularsub-tree. A re-routing node is a node that has been configured to beable to switch to a stand-by alternative path (i.e. a sub-tree) wheneverthe node detects a downstream fault or it is notified by a downstreamnode about a fault event. The node's responsibilities include filteringout fault notifications which are conveyed upstream of the tree (i.e.towards the root node 6, and between the fault and the node 6) in allthose cases in which that node can itself perform the switch to divertthe traffic to alternative path away from the downstream fault. In thisway reaction to dealing with the fault can be kept as local as possibleto the fault, and no other (upstream) node which is not directlyinvolved in the switch operation will know anything about the fault.

On determining that a downstream fault has occurred, the re-routing node4 switches traffic destined for the leaf node 2, to be sent over link 14to a leaf node 10, and the leaf node 10 causes the traffic to be sentover link 15 to the leaf node 2.

As a consequence of the sub-tree switch performed by the node 4, theentire tree has not needed to have been made aware of the fault, and allthe nodes of the tree, except the three nodes, 2, 4 and 10, in thecircle shown as ‘the protection sub-tree’, have seen no increase in theE-tree traffic.

In FIG. 3 a different fault condition has occurred. In this case, it isa fault of the node 4, shown at 30. The stand-by sub-tree which was ableto react to and recover from the fault in FIG. 2, is unable to cope withthis fault condition, and therefore resolving the problem must beeffected closer to the root node 6 of the complete tree. This isequivalent to say that a larger sub-tree, which comprises the sub-treementioned in FIG. 2, has to take action to recover from the fault. There-routing node 20 indicated in FIG. 3, which may be termed the rootnode of the larger sub-tree, is notified and is operative to switch thetraffic to the relevant stand by sub-tree. The re-routing node 20determines that a fault has occurred by way of a fault notificationsignal being sent over the link 11 from the node 8, which is aware ofthe fault. It is to be noted here that node 8 is not a re-routing nodeand so the node 8 is operative to forward the fault signal to theupstream node, node 20. On receiving the signal, the node 20 isconfigured to switch received traffic across a link 27 to a node 21, thenode 21 is configured to send the traffic across the link 28 to the node22, the node 22 is configured to send the traffic across the link 29 tothe node 10 and the node 10 is configured to send the traffic across thelink 15 to the leaf node 2.

A further fault condition is shown in FIG. 4. Compared to the faultconditions described above, the fault occurs closer to the root node 6of the complete tree, and the sub-tree shown in FIG. 3 is not able todeal with it. It should be clear that a larger sub-tree, if available,has to react to this fault, as indicated in FIG. 4. The fault, shown at50, has occurred in a link 36 connecting nodes 20 and 31. On determiningthat a fault has occurred, the (re-routing) node 31 is configured toswitch traffic onto an alternative path to reach the leaf node 2. As isshown in FIG. 4 the alternative path comprises nodes 32, 33, 34, 35, 28and 10 and the links 37, 38, 39, 40, 41, 29 and 15. It should be clearthat a larger sub-tree, if available, has to react to this failure, asindicated in FIG. 4.

In case no sub-tree can be effective to recover from a fault condition,the whole tree may be involved in the protection switch, and in thatsituation the root node 6 of the complete tree performs the necessaryswitch to divert the traffic onto an alternative path.

It will also be appreciated that all the fault conditions which havebeen discussed above have no impact on the leaf nodes on the right endside part of the complete tree.

It will be appreciated that those nodes which are involved in providingan alternative path are suitably configured in an initial set-upprocedure so that diverted traffic arriving at those nodes is sent tothe next node of the alternative path. This may be achieved in aninitial set-up procedure by suitably configuring forwarding tables ofeach of the nodes.

In the above described three fault conditions, three re-routing nodes,4, 20 and 31 at different levels in the tree infrastructure have beenmentioned. Each of those nodes corresponds to what may be termed as arespective sub-root node of three protected sub-trees (i.e. eachsub-tree comprises an active and a stand-by tree), and plays a specialrole in providing an alternative path.

The re-routing nodes are responsible for two possible courses of action:

-   -   to provide fault recovery by way of a switch operation, with no        further notification signal towards upstream nodes        or    -   to delegate protection to an upper sub-tree by sending a fault        notification signal.

It is to be noted that the fault notification signal is conveyedupstream of the tree, because this allows an effective realization ofthe method of re-routing. Provided that the re-routing nodes are not ina fault condition, they are responsible for determining whether a faultnotification signal which they receive in response to a fault condition(however they may also, or alternatively, detect a fault conditiondirectly) can be locally repaired, by switching the traffic onto astand-by sub-tree, of which they are the root, or whether, instead, theyneed to forward a fail notification signal to an upper layer of thetree, which can be a larger sub-tree (which includes their own sub-tree)or the complete tree.

Fault notification signals can conveniently be implemented using knowntechniques, such as Operation Administration and Maintenance (OAM)packets or Control Plane protocols (also depending on the actualtechnology which has been used for the CO-PS network) or by aproprietary message set.

FIG. 5 is a flow diagram showing the overall re-routing method effectedby the network 1. At 100, a fault has occurred in the network. At step101, the fault is detected by a node. If the node is a re-routing nodethen the procedure proceeds to step 104. If the node is not a re-routingnode then, as shown at step 103, a fault notification signal is sentupstream. At the point at which a re-routing node is reached, as shownat step 104, the node determines whether it can recover the fault by aswitching the traffic to an alternative path. If it can, then, as shownat step 105, a switch operation is effected (so as to recover thefault), and no fault notification signal is sent upstream to anothernode. If, however, the re-routing node is unable to recover the fault,then at step 106 it is determined whether the node is in fact the rootnode of the tree. If that node is the root node, then a disjointprotection path is not available. If the node is not the root node thenthe procedure returns to step 103.

FIG. 6 is a flow diagram of the steps taken by a re-routing node. At200, a fault notification signal is sent by a downstream node to there-routing node. On receipt of the signal, step 201 requires that there-routing node determines whether the node is able to recover the faultby switching the traffic to alternative path. If the re-routing nodedetermines that it is so able, then the node performs the step 202 todivert the traffic to an alternative path. This results in the outcome203. If the re-routing node determines that it is not so able, then asshown at steps 204 and 205, the node does not perform a switch operationto divert the traffic to an alternative path, rather it sends a faultnotification signal to an upstream node. From the re-routing node'sperspective this results in the outcome 206. It is to be noted that areason why a re-routing node may not be able to switch traffic onto analternative path is that a link, which forms part of a stand-by treeconnected to the node may be faulty or another node at the opposite endof that link may be faulty. In that scenario an upstream re-routing nodewould be sought, in accordance with the steps shown in FIG. 5. It willbe appreciated that that re-routing nodes can be suitably configured byway of machine-readable instructions, whether in the form of a signal, adata structure or a software product.

There are numerous advantages to the network 1. Fault recovery iscapable of being performed by each sub-tree close to the failure point,so that only the minimum number of components of the network areaffected by the re-routing method. In the case of a failure affecting anode or a portion of the tree where protection locally would requirehigh complexity, fault recovery is “scaled” to an upper level sub-treewhere the fault can be simply bypassed by an alternative path. This maybe viewed as a tree infrastructure composed of a set of sub-treeshierarchically organized. The desired behavior can be deterministicallyachieved because the re-routing nodes are configured to react to afailure in a pre-determined 1:1 fashion, i.e. in the case of faulttraffic is switched from an active sub-tree to a stand-by sub-tree.

In this way, by suitably configuring the protected tree and theprotected sub-trees, thanks to the re-routing method, trafficdisturbances to an E-tree service due to a fault are minimized andconfined locally as far as is possible (or as locally as it isconsidered beneficial by a network operator). Given that every standbysub-tree is configured prior to any fault condition, a switch operationto divert the traffic to an alternative path can take place veryquickly. This is particularly advantageous over known local repairschemes which require a node to be re-configured ‘on the fly’ when afault occurs. Advantageously, the re-routing method performed by thenetwork 1 is not limited to any particular connection orientatedtechnology.

Path protection provided by the re-routing method in the network 1 isadvantageously not limited in its application to any specificconnection-oriented technology.

It is to be noted that even in the case of single points of failure, dueto physical topology constraints active and stand-by trees and sub-treesremain logically separate, so that there is no risk of erroneousforwarding of the traffic.

It will be appreciated that although the above network implements amulticast solution, the method of re-routing traffic implemented by thenetwork 1 is also applicable to unicast scenarios.

The invention claimed is:
 1. A method of re-routing traffic in acommunications network operating in a hierarchical tree structure wherethe network traffic is sent from an ingress node to a plurality ofegress nodes, wherein the ingress node is a root node of the treestructure and the egress nodes are leaf nodes of the tree structure, andwherein in the event of a fault on a path across the network, the methodcomprising: determining, by a first node located between the fault andthe ingress node, whether the first node is capable of autonomouslyswitching traffic to an alternative sub-tree; as a consequence of thefirst node determining that the first node is unable to switch trafficto an alternative sub-tree, sending, by the first node, a faultnotification signal upstream to a second node located upstream of thefirst node, wherein the second node is not the ingress node; as aconsequence of the first node determining that the first node is able toswitch traffic to an alternative sub-tree, determining whether the firstnode is capable of switching traffic from an initial sub-tree to analternative sub-tree which avoids the fault; and as a consequence of thefirst node is determining that the first node is not capable ofswitching traffic to an alternative sub-tree which avoids the fault,sending, by the first node, a fault notification signal upstream to thesecond node, and autonomously determining whether the second node iscapable of switching traffic to an alternative sub-tree which avoids thefault, wherein the communications network is configured to provide anE-tree service to at least part of the nodes of the communicationsnetwork, wherein the ingress node is a node responsible for distributionof the E-tree service within the communications network and the egressnodes are nodes attached to destinations of the E-tree service.
 2. Themethod as claimed in claim 1, wherein at least one further nodedetermines whether the at least one further node is capable of switchingthe traffic to an alternative sub-tree in the event that the second nodedetermines that the second node is not so capable.
 3. The method asclaimed in claim 1, wherein the first node neighbors the fault.
 4. Themethod as claimed in claim 1, wherein the first node determines that thefault has occurred.
 5. The method as claimed in claim 1, wherein thesecond node neighbors the first node.
 6. The method as claimed in claim1, wherein if the first node determines that the first node is able toswitch traffic to an alternative sub-tree, then the fault notificationsignal is not sent to the second node.
 7. The method of claim 1, whereinswitching traffic to an alternative sub-tree which avoids the faultcomprises switching traffic destined for a first leaf node to be sentover a first link to a second leaf node, which in turn causes thetraffic to be sent over a second link to the first leaf node.
 8. Themethod of claim 1, wherein each of the first and second nodes comprisesa sub-root node of a protected sub-tree.
 9. The method of claim 1,wherein each of the egress nodes is attached to a correspondingdestination of an E-tree service.
 10. A communications network operatingin a hierarchical tree structure where the network traffic is sent froman ingress node to a plurality of egress nodes, wherein the ingress nodeis a root node of the tree structure and the egress nodes are leaf nodesof the tree structure, the communications network comprising: aplurality of nodes which are connected by respective links to form atree for traffic across the network, the nodes comprising at least onere-routing node configured to: determine that a fault has occurredbetween the re-routing node and a network egress node, determine if there-routing node is able to autonomously switch traffic onto analternative sub-tree, send a fault notification signal upstream to anupstream node located upstream of the re-routing node as a consequenceof the re-routing node determining that the re-routing node is unable toswitch traffic to an alternative sub-tree, wherein the upstream node isnot the ingress node; determine whether the re-routing node is capableof switching traffic to an alternative sub-tree which avoids the faultas a consequence of the re-routing node determining that the re-routingnode is able to switch traffic to an alternative sub-tree; send a faultnotification signal upstream to the upstream node as a consequence ofdetermining that the re-routing node is not capable of switching trafficto an alternative sub-tree which avoids the fault, and autonomouslydetermine whether the upstream node is capable of switching traffic toan alternative sub-tree which avoids the fault, wherein thecommunications network is configured to provide an E-tree service to atleast part of the nodes of the communications network, wherein theingress node is a node responsible for distribution of the E-treeservice within the communications network and the egress nodes are nodesattached to destinations of the E-tree service.
 11. The communicationsnetwork as claimed in claim 10, wherein the at least one re-routing nodeis configured to switch traffic to at least one alternative subtree. 12.The communications network as claimed in claim 10, wherein the at leastone re-routing node is connected to at least three links.
 13. Thecommunications network as claimed in claim 10, wherein if the at leastone re-routing node determines that the re-routing node is able toswitch traffic to an alternative sub-tree, then the re-routing node isconfigured not to send a fault notification signal is not sent to thesecond node.
 14. The communications network as claimed in claim 10,wherein the communications network is a connection-orientated network.15. The communications network of claim 10, wherein switching traffic toan alternative sub-tree which avoids the fault comprises switchingtraffic destined for a first leaf node to be sent over a first link to asecond leaf node, which in turn causes the traffic to be sent over asecond link to the first leaf node.
 16. A first node for use in acommunications network operating in a hierarchical tree structure wherethe network traffic is sent from an ingress node to a plurality ofegress nodes, wherein the ingress node is a root node of the treestructure and the egress nodes are leaf nodes of the tree structure, thenode comprising: a plurality of hardware interfaces and a processorcollectively configured to: determine that a fault in the network hasoccurred; determine whether the first node is capable of causing trafficto be autonomously switched to an alternative sub-tree, send a faultnotification signal upstream to an upstream node located upstream of thefirst node as a consequence of determining that the first node is unableto switch traffic to an alternative sub-tree, wherein the upstream nodeis not the ingress node; determine whether the first node is capable ofswitching traffic to an alternative sub-tree which avoids the fault as aconsequence of determining that the first node is able to switch trafficto an alternative sub-tree; send a fault notification signal upstream tothe upstream node as a consequence of determining that the first node isnot capable of switching traffic to an alternative sub-tree which avoidsthe fault, and autonomously determine whether the upstream node iscapable of switching traffic to an alternative sub-tree which avoids thefault, wherein the communications network is configured to provide anE-tree service to at least part of the nodes of the communicationsnetwork, wherein the ingress node is a node responsible for distributionof the E-tree service within the communications network and the egressnodes are nodes attached to destinations of the E-tree service.
 17. Thenode as claimed in claim 16, wherein if the processor determines thatthe node is capable of causing traffic to be switched to anothersub-tree, then the processor is configured to cause the faultnotification signal not to be sent.
 18. The first node of claim 16,wherein switching traffic to an alternative sub-tree which avoids thefault comprises switching traffic destined for a first leaf node to besent over a first link to a second leaf node, which in turn causes thetraffic to be sent over a second link to the first leaf node.